Insurers are pressing ahead with a final marketing push to bring as many young, healthy customers as possible onto their rolls and buttress a recent surge in health-law enrollments.

The flood of late sign-ups that helped boost the marketplace total to six million enrollees, a key milestone for the Obama administration, has also brought some insurers an uptick among younger people. But it isn't clear if the trend is broad enough to balance out an earlier skew toward older enrollees, who are more likely to have costly ailments.